<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>欢迎注册</title>
    <link href="css/register.css" rel="stylesheet">
</head>
<body>

<div class="form-div">
    <div class="reg-content">
        <h1>欢迎注册</h1>
        <span>已有帐号？</span> <a href="login.html">登录</a>
    </div>

    <form id="reg-form" action="#" method="get">

        <table>

            <tr>
                <td>用户名</td>
                <td class="inputs">
                    <input name="username" type="text" id="username">
                    <br>
                    <span id="username_err" class="err_msg" style="display: none">用户名已存在</span>
                </td>

            </tr>

            <tr>
                <td>密码</td>
                <td class="inputs">
                    <input name="password" type="password" id="password">
                    <br>
                    <span id="password_err" class="err_msg" style="display: none">密码格式有误</span>
                </td>
            </tr>

            <tr>
                <td>验证码</td>
                <td class="inputs">
                    <input name="checkCode" type="text" id="checkCode">
                    <img src="imgs/a.jpg">
                    <a href="#" id="changeImg">看不清？</a>
                </td>
            </tr>

        </table>

        <div class="buttons">
            <input value="注 册" type="submit" id="reg_btn">
        </div>
        <br class="clear">
    </form>

</div>
<script>
    //1.给用户名输入框绑定 失去焦点事件
    document.getElementById("username").onblur = function () {
        //2.发送ajax请求
        //获取用户名
        var username = this.value;

        //2.1创建核心对象
        var xhttp;
        if (window.XMLHttpRequest) {
            xhttp = new XMLHttpRequest();
        } else {
            xhttp = new ActiveXObject("Microsoft.XMLHTTP")
        }

        //2.2发送请求
        xhttp.open("GET", "http://localhost:8080/ajax-demo/selectUserServlet?" + username);
        xhttp.send();
        xhttp.onreadystatechange = function () {
            //2.3获取响应
            if (this.readyState == 4 && this.status == 200) {
                // alert(this.responseText);
                //判断
                if (this.responseText == "true") {
                    //用户名存在,显示提示信息
                    document.getElementById("username_err").style.display='';
                } else {
                    //用户名不存在，清除提示信息
                    document.getElementById("username_err").style.display= 'none';
                }
            }
        };
    }
</script>
</body>
</html>
如果运行tomcat后css样式和图片加载不出来，可以将maven项目重新打包下，因为在tomcat下面的webapps对应的项目下没有css文件和图片文件